I've got an idea for a West CC Ecosse ride next year from a couple of wee cycle books i've recently acquired. This would suit people coming by car or train.
Start point for car : Gourock. Ride on to Wemyss Bay (9 miles round coast very short stretch on A road after Inverkip).
Ferry to Rothesay (bikes free, fare about £4, 40 min) meet anyone coming by train
Rothesay: optional climb of the "Serpentine" for anyone inclined (boom! boom!). Apparently 11 switchbacks 1.3 Km long avg gradient 8.3%, described in my wee book as "insane"!. Followed by coast road to Rhubodach then
Ferry to mainland (5 min to Colintraive)
Up to Strachur via mostly back roads and CAKE STOP (some hills on this stretch but don't know how hilly?)
Down to Dunoon via Loch Eck (described as easy) and ferry back to Gourock. (30min about £4).
The loop is 75Km plus the bit from Gourock and optional sepentine.
Time required 5 hours riding at easy pace 2 hours for ferries and cake stop. Loads of frequent ferries.
Might be a nice day out? I would hope to recce this route in advance but maybe some of you know some of it?
The Serpentine road has been on my to-do list for a while, but the only bit of the route you describe that I have done is the road from Strachur to Dunoon. It's perfectly fine for riding, although I find it somewhat uninspiring.
The A78 is a nasty road, for me very much something to avoid on a bike, even on the relatively wide bits at Inverkip.